The advent of graphene (K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, “Electric field Effect in Atomically Thin Carbon Films” Science, Vol. 306, No. 5696, pp. 666-669, 2004) and subsequent discovery of its multitude of superior properties, has led to the identification of many other two-dimensional inorganic crystals through the exfoliation of other layered inorganic compounds. Other materials which have been isolated as single or few layer platelets include hexagonal boron nitride, NbSe2, bismuth strontium calcium copper oxide (BSCCO) and MoS2. The nanosheets formed comprise a single or few layers (or sheets) that are stable and can exhibit complementary electronic properties to graphene, such as being insulators, semiconductors and superconductors.
The large variety of two-dimensional atomic crystals isolated in recent years offers a rich platform for the creation of heterostructures which combine several of these materials in one stack. Since, collectively, this class of materials covers a very broad range of properties, the obtained heterostructures can be tuned to focus on particular phenomena, or be used for specific applications (or even to perform multiple functions).
Thus, nanosheets of inorganic two-dimensional materials can be used either alone or in combination with other such materials to form ultrathin electronic devices with astonishing properties. For example, BN and MoS2 have been used in conjunction with graphene to form quantum tunneling transistor heterostructures (WO2012/127245) while MoS2 and WS2 have been used in conjunction with graphene to form photovoltaic heterostructures (WO2013/140181).
To date, heterostructures have generally been produced by micromechanical cleavage of bulk (“three-dimensional”) layered crystals with subsequent dry transfer of each crystal layer. While this technique enables the production of extremely high quality heterostructures, it cannot be applied to the production of such heterostructures on a large scale. Consequently, an alternative method, suitable for mass-production, should be utilised to bring the attractive qualities of such systems to real-life applications.
Liquid-phase exfoliation is a scalable approach for production of nanosheets or flakes of two-dimensional crystals, based on exfoliation of their bulk counterparts via chemical wet dispersion followed by ultra-sonication. This technique offers many advantages in terms of cost reduction, scalability and compatibility with any substrate, including cheap and flexible substrates. Currently this is mostly based on the use of organic solvents such as N-Methylpyrrolidone (NMP) and N,N-dimethylformamide (DMF), which are toxic, expensive and characterized by high boiling points.
The preparation of inkjet printable formulations containing nanosheets of exfoliated inorganic materials (such as graphene, hexagonal boron nitride, NbSe2, bismuth strontium calcium copper oxide (BSCCO), WS2 and/or MoS2) is of particular interest.
Inkjet printing can be used for a wide variety of applications, including the preparation of printed coatings and printed electronic devices. Printed electronic devices are increasingly used in a wide range of commercial applications such as, for example, portable electronic devices, signage, lighting, product identification, flexible electronics, photovoltaic systems, medical equipment, antennas (such as RFID antennas), displays, sensors, thin film batteries, electrodes and many others.
Printed electronics are typically made by printing inks onto a substrate to form the electronic device.
The use of printed electronics has a number of advantages over conventional fabrication processes. In particular, printed conductive and insulative patterns are typically: faster to produce than subtractive processes (such as etching); less wasteful; less hazardous (i.e. use less hazardous chemicals); less expensive than conventional techniques; compatible with a wide range of substrates; simple to implement; and enable the possibility of further post-fabrication processing.
Computer-controlled printer technology also allows for precision printing on a wide variety of substrates, including glass, plastic, or ceramics for electronics or display applications. Inkjet printing involves the placement of small drops of ink onto a substrate surface in response to a digital signal. Typically, the ink is transferred or jetted onto the surface without physical contact between the printing device and the surface. Within this general technique, the specific method by which the inkjet ink is deposited onto the substrate surface varies from system to system, and includes continuous ink deposition and drop-on-demand ink deposition. Ink droplets are ejected by the print head nozzle and are directed to the substrate surface.
In order to be suitable for inkjet printing, the ink needs to meet a number of performance criteria, such as a viscosity within the range 2 to 30 cPs; a surface tension within the range 20 to 50 mN/m (and preferably 28 to 35 mN/m) and a low rate of evaporation at ambient temperatures (to prevent clogging of the printer head). Despite a number of inkjet-printable inks being available, there remains a need for new and improved ink formulations. In particular, there is a need for new and improved aqueous formulations comprising nanosheets of inorganic materials, such as high-quality graphene.
For effective inkjet printing, it is also necessary to have a stable dispersion of the nanosheets of inorganic material within the formulation. To maintain the dispersibility of nanosheet materials such as pristine graphene, it is necessary to use a stabiliser molecule. Examples of stabiliser molecules include certain surfactant, polymers and aromatic “π-π stacking” molecules (e.g. pyrene). However, the presence of excess stabiliser can be undesirable as it may affect the mechanical and electrical properties of the printed ink. There is therefore a need for aqueous ink formulations comprising nanosheets of inorganic materials which comprise minimal quantities of stabiliser.
It is therefore an object of the present invention to provide an ink formulation comprising nanosheets of inorganic materials that is suitable for inkjet printing and the preparation of printed electronics.
The present invention was devised with the foregoing in mind.